1. Field of the Invention
The present invention relates to the technical field of controlling the amount of fuel to be delivered during deceleration or when a key switch is turned off, by providing a vacuum control valve to a fuel cut-off device connecting with the well of a variable choke carburetor having a variable venturi diameter.
2. Description of the Prior Art
As well known, various types of carburetors for automobile engines have been devised and employed. Among them, variable choke or variable venturi carburetors have recently been mounted on a wide range of automobiles ranging from certain types of sport cars to ordinary automobiles because they provide an excellent transient response, are small in height, etc. A typical variable choke carburetor is provided with a fuel cut-off device in order to prevent "run-on" which is the condition where the engine continues to run when the ignition switch is turned off.
There are, however, various problems to be solved in a variable choke carburetor provided with a fuel cut-off device.
One of the problems is that if a run-on preventing fuel cut-off device is employed in order to prevent overheating of a catalyst, after-burning or the like while decelerating, the engine may stall when the clutch is disengaged when decelerating and moreover, discontinuous combustion may occur when the operation changes from deceleration to acceleration or to a normal operating condition.
In a conventional device, a fuel controller 2 provided for a variable choke carburetor 1 has a fuel cut-off device 3 as shown in FIG. 1. When a key switch 4 is turned IC and the engine is idling, a controller 5 detects the rotational speed indicated by a tachometer 6. When the rotational speed goes below a set value the controller 5 actuates a solenoid valve 7 to project its plunger so as to shut off an air bleed 9 which has an air jet 8 at one end and a well 10 at the other.
As is known, the air-fuel ratio when idling is determined by means of fuel 12 ejected after being sucked by a vacuum from a float chamber 11 through a fuel passage 13 and which is metered through the clearance between a metering needle 15, integrally secured to and extending from the head of a suction piston 14, and a metering jet 16 in the well 10.
When decelerating from a normal operating state, when an idling switch 17 is turned IC and the controller 5 detects a rotational speed above the set value, the solenoid valve 7 is turned IC so as to retreat, allowing the air bleed 9 and the well 10 to communicate with each other, thereby controlling the amount of the fuel 12 to be ejected.
However, this decelerating condition and the state in which the key switch 4 and the solenoid valve 7 are turned IC are the same. Moreover, since the diameter of the air jet 8 has been set for an optimum air-fuel ratio, a large amount of air is bled off under the low vacuum resulting when the key switch 4 is off, so that the fuel 12 is reduced to prevent run-on but in the decelerating condition described above, the fuel is reduced mainly in a region in which there is a small amount of intake air. Consequently, the engine may stall when the clutch is disengaged under this condition.